1. Field of the Invention
The present invention relates generally to a receiving apparatus used in a Loran-C navigation system on a basis of a hyperbolic electromagnetic navigation technique, and relates particularly to a Loran-C signal receiving apparatus capable of accurately and speedily identifying a particular cycle of a carrier wave in each received Loran-C pulse.
2. Description of the Prior Art
A Loran (Long range navigation)-C system employs a chain comprising one master transmission station and two or more secondary transmission stations.
The master transmission station transmits nine Loran pulses as shown by M of (a) in FIG. 1.
Each secondary transmission station transmits eight Loran pulses as shown by S.sub.1 and S.sub.2 of (a) in FIG. 1. Each transmission station generates the above-described Loran pulses at a pulse repetition rate prescribed for each chain. In addition, each secondary transmission station generates its secondary station Loran pulses with a coding delay with respect to the transmission pulse of the master transmission station distinct from other secondary transmission stations.
Hence, in the Loran-C signal receiving apparatus, the difference in distance to the two fixed points represented by the master and each secondary transmission station can be obtained from the delay between receipt of the secondary transmission station pulses and the master transmission station pulses so that the location of the Loran-C signal receiving apparatus is specified by the intersection of Loran hyperbolics drawn for each of the two sets of two fixed points.
In the above-described Loran-C signal receiving apparatus, a particular cycle of the carrier wave in each received pulse (generally, the third cycle of the carrier wave) is tracked in order to measure the reception delay time of the pulses from the secondary transmission stations with respect to those from the master transmission station.
The carrier wave Ca of the above-described Loran-C signal has a frequency of 100 killohertz and hence a period of 10 microseconds as appreciated from (c) in FIG. 1 drawn on an expanded time axis.
A conventional Loran-C receiving apparatus performing this function is disclosed in Japanese Patent Application Open Nos. 55-2938, No. 55-6261, and Patent Publication No. 56-2312. In addition, the general concepts of Loran-C receiving apparata are disclosed in U.S. Pat. No. 4,268,830 to Lester R. Brodeur filed on Apr. 9, 1979.
The method for identifying the third cycle of the carrier wave Ca in conventional Loran-C receiving apparata is that once the Loran-C pulse LP is received, a sampling pulse is generated in synchronization with the pulse repetition rate (for example, in the Japanese Maritime Area 99.7 milliseconds) of the Loran-C pulse LP. If the time-integrated value of the signal amplitude sampled in response to sampling pulses exceeds a predetermined threshold value, the corresponding phase of the signal is determined to be one of the peaks P.sub.1, P.sub.2, and P.sub.3 of the carrier wave Ca. After detecting the accompanying peaks of the carrier wave, the third cycle of the carrier wave Ca in each pulse is identified by determining that the third sampled value in excess of the threshold is the peak of the third cycle.
However, since conventional Loran-C receiving apparatus identifying the third cycle of the carrier wave by detecting the peak of the carrier wave, due to the difficulty in detecting first and second peaks accurately in cases where the S/N ratio (Signal-to-Noise) is excessly low due to external noise, the third cycle may easily be improperly identified.
If the above-described Loran-C receiving apparatus is mounted in an automotive vehicle in order to monitor the position of the moving vehicle, since the Loran-C signal will be highly attenuated in urban or mountainous areas, the S/N ratio of the received signal will often be extremely low. Therefore, the above-described problem would be significant.